The control and/or elimination of undesirable impurities and by-products from various manufacturing operations has gained considerable importance in view of the potential pollution such impurities and by-products may generate. One conventional approach for eliminating or at least reducing these pollutants is by thermal oxidation. Thermal oxidation occurs when contaminated air containing sufficient oxygen is heated to a temperature high enough and for a sufficient length of time to convert the undesired compounds into harmless gases such as carbon dioxide and water vapor.
Control of web drying apparatus, including flotation dryers capable of contactless supporting and drying a moving web of material, such as paper, film or other sheet material, via heated air issuing from a series of typically opposing air nozzles, requires a heat source for the heated air. Additionally, as a result of the drying process, undesirable volatile organic compounds (VOCs) may evolve from the moving web of material, especially where the drying is of a coating of ink or the like on the web. Such VOCs are -mandated by law to be converted to harmless gases prior to release to the environment.
Prior art flotation drying apparatus have been combined with various incinerator or afterburner devices in a separated manner in which hot, oxidized gases are retrieved from the exhaust of the thermal oxidizer and returned to the drying device. These systems are not considered fully integrated due to the separation of oxidizer and dryer components and the requirement of an additional heating appliance in the drying enclosure. Other prior art systems combined a thermal type oxidizer integrally within the dryer enclosure, also utilizing volatile off-gases from the web material as fuel. However, this so-called straight thermal combustion system did not utilize any type of heat recovery device or media and required relatively high amounts of supplemental fuel, especially in cases of low volatile off-gas concentrations. Still other prior art apparatus combined a flotation dryer with the so-called thermal recuperative type oxidizer in a truly integrated fashion. One disadvantage of these systems is the limitation of heat recovery effectiveness due to the type of heat exchanger employed, thus preventing extremely low supplemental fuel consumption capabilities and often precluding any auto-thermal operation. This limitation in effectiveness results from the fact that a heat exchanger with high effectiveness will preheat the incoming air to temperatures high enough to cause accelerated oxidation of the heat exchanger tubes which results in tube failure, leakage, reduction in efficiency and destruction of the volatiles. In general, the thermal recuperative type device has a reduced reliability of system components such as the heat exchanger and burner due to the exposure of metal to high temperature in-service duty.
Yet another fully integrated system utilizes a catalytic combustor to convert off-gases and has the potential to provide all the heat required for the drying process. This type system can use a high effectiveness heat exchanger because the presence of a catalyst allows oxidation to occur at low temperatures. Thus, even a high efficiency heat exchanger can not preheat the incoming air to harmful temperatures. However, a catalytic oxidizer is susceptible to catalyst poisoning by certain components of the off-gases, thereby becoming ineffective in converting these off-gases to harmless components. Additionally, catalytic systems typically employ a metal type heat exchanger for primary heat recovery purposes, which have a limited service life due to high temperature in-service duty.
For example, U.S. Pat. No. 5,207,008 discloses an air flotation dryer with a built-in afterburner. Solvent-laden air resulting from the drying operation is directed past a burner where the volatile organic compounds are oxidized. At least a portion of the resulting heated combusted air is then recirculated to the air nozzles for drying the floating web.
U.S. Pat. No. 5,210,961 discloses a web dryer including a burner and a recuperative heat exchanger.
EP-A-0326228 discloses a compact heating appliance for a dryer. The heating appliance includes a burner and a combustion chamber, the combustion chamber defining a U-shaped path. The combustion chamber is in communication with a recuperative heat exchanger.
In view of the high cost of the fuel necessary to generate the required heat for oxidation, it is advantageous to recover as much of the heat as possible. To that end, U.S. Pat. No. 3,870,474 discloses a thermal regenerative oxidizer comprising three regenerators, two of which are in operation at any given time while the third receives a small purge of purified air to force out any untreated or contaminated air therefrom and discharges it into a combustion chamber where the contaminants are oxidized. Upon completion of a first cycle, the flow of contaminated air is reversed through the regenerator from which the purified air was previously discharged, in order to preheat the contaminated air during passage through the regenerator prior to its introduction into the combustion chamber. In this way, heat recovery is achieved.
U.S. Pat. No. 3,895,918 discloses a thermal rotary regeneration system in which a plurality of spaced, non-parallel heat-exchange beds are disposed toward the periphery of a central, high-temperature combustion chamber. Each heat-exchange bed is filled with heat-exchanging ceramic elements. Exhaust gases from industrial processes are supplied to an inlet duct, which distributes the gases to selected heat-exchange sections depending upon whether an inlet valve to a given section is open or closed.
It would be desirable to take advantage of the efficiencies achieved with regenerative heat exchange in air flotation dryers.